Electron beam deflecting means



C3088 REFERENCE SEARCH Mn.

Dec. 9, 1969 c. w. HANKs ELECTRON BEAM DEFLECTING MEANS 2 Sheets-Sheet lFiled July 26, 1967 n m. f .Ii c. ,1 n..

I N E' N TO I2 CHA/25s h/ HAM/s WM M4 Dec. 9, 1969 c. w. HANKs 3,483,417

ELECTRON BEAM DEFLECTING MEANS Filed July 26, 1967 2 Sheets-Sheet 2INVENTOR @mais /L/ #4M/s ATTOENEYS 3,483,417 ELECTRON BEAM DEFLECTINGMEANS Charles W. Hanks, Orinda, Calif., assignor to Air Reduc tionCompany, Incorporated, New York, NSY., a corporation of New York FiledJuly 26, 1967, Ser. No. 658,579 Int. Cl. H01j 3/20, 3/32, 23/10 U.S. Cl.313-156 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates tomean-s for deecting an electron beam. The invention is particularlyapplicable to use in a high vacuum, electron beam furnace.

High vacuum electron beam furnaces have been used for some time in thevacuum processing of various materials. Such furnaces are utilized, forexample, in the melting and casting of metallic ores to obtainrelatively pure metals or alloys. Such furnaces are also used in themelting of materials other than metals, such as ceramics and plastics,and are frequently used to produce vapors of metals and other materialsfor deposition on a substrate.

Electron beam furnaces utilize one or more electron beam gun assembliesfor producing high energy electron beams and directing such beams to atarget to be heated. It has been vfpund that the use of transversemagnetic fields for deiiecting the beam through a curving path permitsthe electifon gun to be placed in a location wherein it is less likely?.to be damaged by direct condensation of evaporant or by ion bombardment.Focusing'action may be achieved in the direction transverse to the planeof the curving electron beam path (lateral focusing) where the lines offlux in the transverse magnetic eld are bowed to produce a barrelshapedmagnetic eld.

One way ofproducing bowed ux lines is by disposing the polepiecem'surfaces between which the eld is established at an a'iigle withrespect to each other. Such an.

expedient may be satisfactory for larger applications, but whereequipment is to be compact or where a large number of adjacent electronbeams are to be used, angled pole piece surfaces may occupy too muchspace. Under some circumstances, it may be desirable to establish verydeeply bowed magnetic flux lines to produce a sharp lateral focusingaction on the beam. However, it is sometimes not possible to achieve adeeply bowed field without making the magnetic eld gradient along theradius of the curving beam path so large that outer elements of the beamare lost because of a weak field. Because of such losses, the heatingprocess may become too ineicient.

It is therefore an object of the invention to provide compact means fordeecting and laterally focusing an electron beam.

Another object of the invention is to provide means for deflecting anelectron beam wherein the beam is laterally focused without excessiveloss of outer electrons of the beam.

A further object of the invention is to provide means for establishing atransverse magneticeld, for deflecting an electron beam, wherein thelines offorce are deeply bowed.

3,483,417 Patented Dec. 9, 1969 S;ill another object of the invention isto provide an electron beam gunl assembly having improved focusingcharacteristics and which is low in cost and simple of construction.

Other objects of `the invention will become apparent to those skilled inthe art from the following description taken in connection with theaccompanying drawings wherein: yI

FIGURE l is aftop view of an electron beam gun assembly constructed inaccordance with the invention and illustrating a crucible for containinga target material to be heated;

FIGURE 2 is a sectional view taken along the line 2 2 of FIGURE l;

FIGURE 3 is a schematic end view illustrating the characteristics of avtransverse magnetic eld established by means similar 'to those utilizedin the electron beam gun assembly of FIGURES 1 and 2; and

FIGURE 4 is a schematic side view of bodiment of the invention. .p

Very generally, 'an electron beam gun assembly in accordance with theinvention includes a pair of spaced oppositely polarized main polepieces adapted to -pro duce a magnetic field having lines of fluxextending-between the pole pieces transversely of the path of anelectron beam for deecting the beam. An elongated auxiliary pole pieceis secured to each main pole piece so as to have the same polarity. Theauxiliary pole pieces extend generally transversely of the beam path andare of a. size which causes them to magnetically saturate. The auxiliarypole pieces are of a length such that their free ends are separated by agap such as` to produce lines of ux which are bowed substantially in theregion proximate the gap and in the beam path.

Referring now more particularly to FIGURES 1 and 2, molten material 1v1,contained within a crucible 12, is melted by bombarding the material byan electron beam, as explained below. The crucible is supported on apedestal 13 which rests upon a base plate 14. The crucible is disposedwithinI la high vacuum environment and is provided with coolant passagesl16 for cooling the crucible during melting operations. Accordingly, askull of solid a further emmaterial, not illustrated, may form betweenthe molten material 11 and the interior wall of the crucible 12 toprevent interaction between the crucible and the molten material.

An electron beam for heating the molten material 11 in the crucible 12is produced by an electron beam gun 17. The gun 17 includes an elongatedemitter 18 c0mprised of tungsten or a material having similar propertiesand, when a heating current is passed therethrough, produces freeelectrons. The emitter 18 is supported, by suitable means notillustrated, in recess 19 formed. in a focusing electrode 21. Theunillustrated emitter supports may also provide electricalcontact forpassing the heating current through the emitter.

The electrons in the beam are accelerated by the acn celeratingelectrode or anode 22. The anode 22 consists of a metal plate having anopening 23 therein through which the beam passes. One edge 24 of themetal plate 22 is turned downward and is bolted to a vertical supportplate 26. The support plate has two inwardly extending legs 27 and 28which are bolted to the base plate 14. The focusing electrode 21 and theemitter 18 are maintained at a negative potential with respect to theaccelerating electrode 22, and the recess 19 is shaped such that thefree electrons produced by the emitter are directed out of the open endof the recess in the shape of a beam. Because of the elongated shape ofthe emitter 18 and the elongated shape of the anode opening 23, the beamis in the form of a ribbon or a narrow rectangle as it leaves the anodeopening. The edges of the ribbon shaped beam are indicated by thedash-dot lines 32 and 33 in FIGURE 1. f

After leaving the anode opening 23, the beam is deflected by atransverse magnetic fieldfto pass through a curving path and make achange in direction of slightly less than 180. In the apparatus ofFIGURES l and 2, they main magnetic field is establishedbetween a pairof vertically extending pole pieces 36 and 37. The pole pieces 36 and 37extend upwardly from the base plate 14, and suitable connection is madeto the pole pieces to establish a transverse magnetic fieldtherebetween.

When an electron beam passes through a transverse' field having bowedlines of ux so that the field approximates a barrel shape, lateralfocusing of the beam occurs. For a very short emitter which produces acorrespondingly small width electron beam, the. flux lines of thetransverse magnetic eld cannot be made to bow sharply-venough to focusthe small width beam without making the magnetic field gradient so greatthat the outer parts of the field are relaitvely weakl" The weak part ofthe field may cause some of the electrons of the electron beam to belost, and a fall-ofir in heating efficiency results.

The invention provides a deeply bowed magnetic field capable ofproviding satisfactory lateral focusing in systems wherein thetransverse magnetic field is relatively large with respect to the crosssectional area of the electron beam. In accordance with the invention, amagnetic field having strongly or sharply bowed lines of flux isestablished near the anode hole 23. In this region, the electron beam isslightly divergent but still of small cross sectional area. Such asharply bowed local field results inI a correction of the beamdivergence depending upon the sharpness of the bowing in the lines ofliux of the field. The amount of correction can be made to match thedivergence of the electron beam and start it gradually to a focal regionat a desired distance. Thus, as may be seen in FIGURE 1, the sharplybowed local field represented by the flux lines 31 causes the edges 32and 33, respectively, of the ribbon-shaped electron beam to converge ata focal region 34.

In order to establish the local strongly bowed field near the opening23, a pair of auxiliary pole pieces 38 and 39 extend inwardly from themain pole pieces 36 and 37. The auxiliary pole pieces are mounted onmounting blocks 41 and 42, respectively, which mate in grooves formed inthe main pole pieces. Because of the partcular geometry of theillustrated system, the auxiliary pole pieces 38 and 39 extend inwardangularly toward the emitter 18. In other systems, however, theauxiliary pole pieces may be axially aligned or otherwise disposedaccording to the particular needs of the system. The auxiliary polepieces 38 and 39 are of a size which saturate, deriving their magneticfield from the main pole pieces 36 and 3'7, and the strongly bowed localmagnetic field is established extending across the gap between the tipsof the auxiliary pole pieces.

Depending upon the emitter length and other considerations, the focusingeffect can be varied by a change in the spacing between the ends of theauxiliary pole pieces 38 and 39. In the particular illustrated system,the focal region 34 may be attained to coincide with the surface of thetarget where a spacing between the tips of the anxiliry pole pieces 38and 39 is about one to two times the width of the beam at that point andwhere the beam path length in the main magnetic field is about two tofour times the length of the gap between the pole pieces 38 and 39. Byproviding sets of interchangeable pole pieces 38 and 39 with variouslengths, or by making the effective length of the pole pieces variableby a threaded type ad justment (such as illustrated in FIGURE 3 by thethreaded opening 43 and the threaded pole piece end 44) an adjustablebeam size is possible.

It may be noted from FIGURE 3 that, in addition to the production of alocal strongly bowed magnetic field, the use of auxiliary pole pieces asdescribed causes a general increase in the bowing of all field lines inthe main magnetic field. In FIGURE 3, a transverse magnetic field isestablished between the po'e pieces 46 and 47. Were the auxiliary polepieces 48 and 49 not present, the lines of ux between the main poepieces would appear as the dotted lines 51. As a result of the placementof pole pieces, however, the lines of flux in the main magnetic'fieldare more deeply bowed as indicated by the solid lines S2. The localstrongy bowed field is indicated in FIGURE 3 by the dotted lines 53.

A further advantage accruing from the use of auxiliary pole pieces asdescribed is in the ability to achieve bowed flux lines with compactapparatus. It is unnecessary to make the main pole pieces of irregularshape, or to dispose them at an angle with respect to each other, inorder to produce bowed flux lines. Thus, for example, several electrongun assemblies may be placed immediately adjacent each other and occupya smaller space. Moreover, structural aspects are more simple when thepole pieces may be a pair of parallel plates.

The use of two sets of auxiliary pole pieces is shown in FIGURE 4. Themain pole pieces, one of which is indicated at 54, may be similar tothose in FIGURES l and 2. The pole pieces are used to deflect anelectron beam produced by a suitable electron gun 56 to impinge upon thesurface of material in a crucible 57. Two pairs of axially aligned polepieces 58 and S9 are utilized. The auxiliary pole pieces 58 and 59extend inwardly from the main pole pieces and terminate to leav'e a gapof the desired size. A control circuit or similar arrangement 61 isconnected to the main pole pieces in order to vary the strength of themagnetic field established therebetween. By increasing the strength ofthe magnetic eld, the total amount of deflection of the electron beammay be increased, and the two extreme positions of the beam areindicated in FIGURE 4 by the solid lines 62 and by the dotted lines 63,respectively. By suitably varying the strength of the magnetic field,the beam may be swept between the two extremes across the surface lofthe molten material in the Crucible 57 to melt the material in a desiredmanner.

In order to insure that the beam will be suitably laterally focused inall its variable positions, the use of the two sets of auxiliary polepieces 58 and S9 is made. Lateral focusing of the outer beam 62 isaccomplished by the pair of auxiliary pole pieces 59, whereas lateralfocusing of the beam in the position 63 is accomplished by the pair ofauxiliary pole pieces 58. Naturally, more than two pairs of pole pieces,suitably positioned, may be utilized, however, in the illustratedembodiment, the size of the local fields established by the pole pieces58 and 59 may be sufficient to cover the entire amount of deflection ofthe beam between the two positions 62 and 63. Therefore, 'only two pairsmay be satisfactory.

Because of the production of local strongly bowed magnetic fields, andbecause of the general increase in the bowing of all field. lines in themain magnetic field as the result of the use of auxiliary pole pieces asshown, considerable flexibility in the establishment of transversemagnetic fields for deflecting electron beams is provided. For example,if auxiliary poles are placed near the center of mutually parallel mainpole pieces each of which is generally in the shape of three-quarters ofa circular disc, the converging effect on an electron beam passingthrough a field established between such pole pieces is almost identicalto that provided by a pair of conically-shaped pole pieces. Thus, theuse of saturating auxiliary pole pieces in connection with variousshapes of `main pole pieces makes possible the production of complexmagnetic fields without the necessity for complex configuration in thepole pieces. It also facilitates experimentation, since complex magneticfields with local variation may be obtained and varied empiricallyrather than by the design and machining of more costly shaped polepieces for each type of field.

It may therefore be seen that the invention provides an electron beamgun assembly including means for deecting an electron beam which achievesuperior focusing at low cost and with relatively simple construction.The invention is of particular advantage in focusing and deflectingelectron beams of relatively small cross section.

Various embodiments of the invention other than those shown anddescribed herein will beco-me apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such otherembodiments, and modifications thereof, are intended to fall within thescope of the appended claims What is claimed is:

1. Means for deliecting an electron beam, including a pair of spacedoppositely polarized main pole pieces adapted to produce a main magneticfield having lines of flux extending ybetween said pole piecestransversely of the beam path, and a pair of elongated auxiliary polepieces for producing a local magnetic iield, each being secured to arespective one of said main pole pieces to have the same polarity, saidauxiliary pole pieces projecting inwardly from said main pole piecestoward each other to extend generally transversely of the beam path,said auxiliary pole pieces being of a size with respect to said mainpole pieces so as to saturate magnetically and produce a general bowingof the flux lines of the main magnetic field, said auxiliary pole piecesbeing of a length such that their free ends are separated by a gap toproduce lines of flux in the local iield which are bowed substantiallymore than those of the main ield in the region proximate said gap and inthe beam path.

2. Means in accordance with claim 1 wherein the distance each of saidauxiliary pole pieces projects from said main pole piece to which it isattached is adjustable.

3. Means according to claim 2 wherein said auxiliary pole pieces arethreaded into correspondingly threaded openings in said main pole piecesand are adjustable therein.

4. Means in accordance with claim 1 wherein at least one further pair ofelongated auxiliary pole pieces are provided for producing a secondlocal magnetic field, each being secured to a respective one of saidmain pole pieces to have the same polarity, said further pair ofauxiliary pole pieces projecting inwardly from said main pole piecestoward each other to extend generally transversely of the beam path,said further auxiliary pole pieces being of a size with respect to saidmain pole pieces so as to saturate magnetically and produce a generalbowing of the iiux lines of the main magnetic field, said further pairof auxil= iary pole pieces being of a length such that their free endsare separated by a gap to produce lines of flux in the local field whichare bowed substantially more than those of the main eld in the regionproximate said gap, means for varying main lield strength between saidmain pole pieces to change the beam path between at least two positions,said further pair of auxiliary pole pieces being positioned such thatthe region proximate said gap is in one of said beam path positions,

5. An electron beam gun assembly for producing and deflecting anelectron beam, including an electron beam source, a pair of spaced4oppositely polarized main pole pieces adapted to produce a main magneticiield having lines of ilux extending between said pole piecestransversely of the beam path, and a pair of elongated auxiliary polepieces for producing a local magnetic ield, each being secured to arespective one of said main pole pieces to have the same polarity, saidauxiliary pole pieces projecting inwardly from said main pole piecestoward each other to extend generally transversely of the beam path,said auxiliary pole pieces being of a size with respect to said mainpole pieces so as to saturate magnetically and produce a general bowingof the iiux lines of the main magnetic field, said pole pieces being ofa length such that their free ends are separated Iby a gap to producelines of iiux in the local'iield which are bowed substantially more thanthose of the main `iield in the region proximate said gap and in thebeam path.

6. An electron beam gun assembly in accordance with claim S wherein saidauxiliary pole pieces are positioned proximate the electron beam sourceand wherein said gap is larger than the corresponding cross sectionaldimension of the beam thereat.

7. An electron beam gun assembly in accordance with claim 6 wherein saidgap is about one to two times the corresponding cross sectionaldimension of the beam thereat.

References Cited UNITED STATES PATENTS 3,132,198 5/1964 Du Bois et al13-31 X 3,202,794 8/1965 Shrader et al 13-31 X 3,235,647 2/1966 Hanks13-31 3,390,222 6/1968 Anderson 13--31 I AMES W. LAWRENCE, PrimaryExaminer R. F. HOSSFELD, .Assistant Examiner U.S. C1. X.R.

